1. Field of the Art.
This invention relates to a digital signal reflection attenuation device for digital transmissions and, more particularly, to a reflection attenuation device for digital transmission lines that carry digital signals, such as time division multiplex (TDM) signals.
2. Background of the Prior Art.
Transmission of digital signals, such as 350 kHz TDM signals, requires placing the signal on a transmission line with a line driver and then receiving this signal at the other end of the transmission line with a receiver. The received signal must accurately reproduce the originally transmitted signal, or else the informational content of the signal will be lost.
The prior art, to ensure the accurate reception of the transmitted signal by the receiver, painstakingly makes sure that the characteristic impedance Zo of the receiver matches the characteristic impedance (generally resistive) of the transmission line. With a proper match, reflections do not occur and the received signal can be accurately reproduced.
However, matching the characteristic impedance of the transmission line at the receiver is costly in terms of power consumption. If the line is not matched, reflections occur and signals on transmission lines becomes distorted. This distortion is due to the superposition of reflected signals and transmitted signals. The signal will be reflected in phase if the end impedance is greater than Zo, and the signals will be reflected in reverse phase if the end impedance is less than Zo. At the transmitter end the line is essentially pinned to a.c. ground through the low driver impedance, so reflections are reversed at the transmitter. As multiple reflections occur at both the driver and the receiver, the distortion increases tremendously. In effect, the transmission line acts as a voltage multiplier in that non-sinusoidal driving functions are converted to sine waves. For properly terminated transmission lines, rounding of waveforms is caused by frequency dispersion: in general, the propagation velocity is less for the higher frequencies.
An example of a worst case match of characteristic impedance is the termination of a transmission line in an open circuit. For a 100 foot flat ribbon cable transmission line having a ground line, a signal line, and a 5 Volt supply line, a series of 350 kHz, 5 Volt, square wave pulses transmitted along the signal line will result in a signal at the terminal end that is sinusoidal and can have peak to peak voltages of 70 Volts. The difference in the transmitted and received signals is graphically illustrated in FIGS. 1A and 1B.
Furthermore, although many different types of transmission lines are known, such as optical fibers and coaxial cables, these types of transmission lines are very expensive. Thus, many applications that could benefit from digital communications are not even considered because of their expense. For example, the cost of installing optical fibers or coaxial cables in a house would be prohibitive. Also, the instability of the characteristic impedance in a twisted wire pair makes its use as a longer length transmission line difficult.
Another problem with past digital transmissions lines is that they have required line drivers having a large power handling capacity, due to the resistive terminations at the receiving nodes of the transmission line. This resistive termination has, in the past, been necessary to match the characteristic impedance of each receiver and the transmission line. As the number of receivers attached to a transmission line increases, the resistance of each receiver acts as a separate load on the driver, thereby requiring line drivers having large power handling capacity, especially as the number of taps on the transmission line is increased. The parallel resistive terminators and the series line resistance act as ladder attenuators, which can greatly reduce both the a.c. and d.c. levels down the line. In turn, this reduces the signal to noise (S/N) ratio of the transmitted signal and requires the use of more expensive digital receivers so that the received signals can be accurately reproduced.
Thus, an inexpensive transmission line that does not suffer from unattenuated reflections and allows use of a driver having a smaller power handling capacity is necessary. More generally, a way of attenuating line reflections for any type of transmission line is needed.